The reversible oxidation of molecular hydrogen is a reaction that is of paramount importance in the metabolism of a large number of bacteria. The x-ray crystal structure of the iron-only hydrogenase from the anaerobic soil microorganism Clostridium pasteurianum has been determined by MAD phasing to 1.8 E resolution (Science, in press). The active site cluster or "H cluster" is structurally unprecedented among previously characterized biological FeS clusters contributing new insights into how this enzyme functions that could provide the foundation for future projects involving H2 as an energy source.